1. Field of the Invention
The present invention relates to a method of working paperboard blanks and, more particularly, to a method of offsetting tool heads to form a panel line on a paperboard blank.
2. Related Prior Art
Boxes, such as those made from corrugated board or paperboard, are generally made by cutting a paperboard blank to the desired outside dimensions of the various walls and flaps, scoring the blank to form a crease line where the box is to be folded and slotting the blank to form the various flaps of the box. The crease lines and slotting lines are often referred to as "lines of working" on the paperboard blank. In conventional machines for forming box blanks, the tool heads for slotting and scoring the blanks are mounted in mating positions on successive pairs of shafts wherein each pair of shafts is oriented with one shaft above the other. Thus, the slotting and scoring operations take place serially as the blank is fed along a horizontal feed path into a nip as defined by blades or profiles on each set of mating tool heads on paired shafts.
Accurate scoring is critical to the squareness and joint gap control of the resulting box. The crease line formed by scoring must be wide enough to allow a fold to occur without rolling and thereby creating a false fold line. Further, the crease line should not be so narrow as to create cracking of the inside liner of the corrugated board, thereby weakening the box. However, a crease line which is too wide will lack adequate strength as it will tend to roll. The proper width of the crease line depends upon the thickness or caliper of the paperboard blank. For light weight papers and thin blank calipers, only a narrow crease line is required. However, to ensure proper box folding, heavy weight papers and thick blank calipers require a wider crease.
In the prior art methods associated with typical box forming machines, each crease line is formed by a series of mating scoring heads aligned in the direction of board travel wherein the successive scoring profiles share a common center axis. This prior art process is often referred to as an in-line arrangement of scoring heads. Each successive downstream set of mating scoring heads produces a score line superposed over the preceding score line such that each score line shares a common longitudinal axis resulting in a crease line centered on such axis.
In order to change the width of the resulting crease line in this prior art process, each set of mating scoring heads along the common longitudinal axis is replaced with another set of heads having scoring profiles of a different width. A plurality of scoring heads having different width profiles are typically located along each supporting shaft. When crease lines of a different width are desired, the scoring heads must be manually repositioned along their respective shafts such that the scoring profiles of desired width are aligned for engaging the corrugated board. Further, attempts to produce crease lines of varying widths in a conventional in-line arrangement of scoring heads has often resulted in increased crushing pressures and cracking, especially when light weight liners or thin board calipers are used.
In response to the limitations of traditional scoring or creasing methods, "dual" or "progressive" scoring has been proposed wherein crease lines are formed by a primary pair of mating scoring heads followed by a secondary pair of mating scoring heads, each pair having scoring profiles of a different shape to ensure a well defined crease line. However, progressive scoring utilizes a traditional in-line arrangement of scoring profiles in that the profiles of both the primary scoring heads and secondary scoring heads share a common center axis thereby requiring the replacement of scoring heads when a change in crease line width is desired.
In addition to forming crease lines of desired width, conventional methods of working paperboard blanks include forming slotting lines by passing the blank between mating slotting heads. Each slotting line is usually associated with a crease line to define a single panel line wherein the slotting line typically shares a common longitudinal axis with the crease line. It is sometimes desirable, however, to offset the slotting line from its associated crease line such that the flaps of the resulting box may be folded in a particular configuration. When such offsetting is desired, the prior art methods require that the slotting and scoring heads be manually repositioned along their respective shafts such that the center axes of the slotting and scoring lines are spaced apart by the desired offset.
This manual repositioning of tool heads has proven to be a time consuming task resulting in decreased production efficiency. Typically, the machine operator must physically enter the working area of the machine to manually disengage the tool heads from their respective shafts and then push the tool heads to their new positions.
Accordingly, there is a need for a method of scoring paperboard blanks to form a crease line wherein the width of the crease line may be easily varied without requiring the replacement of the scoring tool heads. In addition, there is a need for such a method for producing a well-defined crease line without cracking the paperboard blank. Further, there is a need for a method of displacing tool heads to form offset lines of working on a paperboard blank without requiring the manual repositioning of the tool heads.